1. Field of Invention
The present invention relates to a process using radiofrequency microwave energy to enhance wet oxidation of materials often of a hazardous nature.
2. Background
Wet air oxidation (WAO), a common subcategory of wet oxidation, is the oxidation of chemical substances that often represent hazardous materials whereby the oxidation products are usually of a nonhazardous nature. Generally such chemicals are organic substances or materials, often referred to just as organics and restricted to only non-biological material, and represent a constituent of contaminated water, so the oxidation reaction occurs in the presence of water molecules.
WAO is an attractive pollution-control process because it is an enclosed process and has very limited interaction with the environment, and further the waste or contaminants are destroyed, instead of merely being broken down into another form of pollution. In most instances the end products are carbon dioxide and water; however, in some cases various carboxylic acids are formed.
In many instances the source of oxidation is an air stream thus leading to the designation WAO, but pure oxygen is potentially usable. In order for oxidation to occur in the WAO process the temperature and pressure must be elevated. Typical temperatures employed are from 150-325.degree. C. while pressures are typically in the range of 2000-20,000 kPa. In most WAO processes agitation is employed to transfer oxygen from the gas phase to the liquid phase where the oxidation reaction occurs at these elevated conditions. A common usage for WAO is the treatment of high-concentration wastewater where a 70+ percent reduction in chemical oxygen demand (COD) occurs after about one hour at approximately 250.degree. C. and 2.5 MPa. Substituting pure oxygen for air increases the COD removal efficiency. For a summary of some typical WAO uses see Kiely, Environmental Engineering, pp 603-605, 728-730, McGraw-Hill, N.Y. 1997.
Conventional WAO has been improved by the utilization of a catalyst which speeds up the oxidation process and generally allows the use of lower temperatures and pressures. One common catalyst is platinum. Another is an iron-based catalyst, and used with wastewater, this is called the Loprox process. For instance, see Williams, "The Loprox Route to Wet Oxidation," Volume 4, pp 28-29, World Water and Environmental Engineering,1997.
In the subject case the oxidation of the hazardous waste material hydrazine is a primary consideration along with further organics, such as various solvents which may contain, or have dissolved or mixed compounds containing, chlorine molecules.
Hydrazine or its derivatives, monomethyl hydrazine and unsymmetrical dimethyl hydrazine, are common spacecraft propellants for use in missiles, rockets, and space launch vehicles. When used as such an astronautics fuel, nitrogen tetroxide is the most common oxidizer. However around space launch areas, much wastewater containing dilute hydrazine occurs and must be environmentally cleaned up. Microwave enhanced wet oxidation appears as a favorable process to perform this clean up task.
Quantum radiofrequency (RF) physics is based upon the phenomenon of resonant interaction with matter of electromagnetic radiation in the microwave and RF regions since every atom or molecule can absorb, and thus radiate, electromagnetic waves of various wavelengths. The rotational and vibrational frequencies of the electrons represent the most important frequency range. The electromagnetic frequency spectrum is usually divided into ultrasonic, microwave, and optical regions. The microwave region is from 300 megahertz (MHz) to 300 gigahertz (GHz) and encompasses frequencies used for much communication equipment. For instance, refer to Cook, Microwave Principles and Systems, Prentice-Hall, 1986.
Often the term microwaves or microwave energy is applied to a broad range of radiofrequency energies particularly with respect to the common heating frequencies, 915 MHz and 2450 MHz. The former is often employed in industrial heating applications while the latter is the frequency of the common household microwave oven and therefore represents a good frequency to excite water molecules. In this writing the term "microwaves" is generally employed to represent "radiofrequency energies selected from the range of about 500 to 5000 MHz", since in a practical sense this total range is employable for the subject invention.
The absorption of microwaves by the energy bands, particularly the vibrational energy levels, of atoms or molecules results in the thermal activation of the nonplasma material and the excitation of valence electrons. The nonplasma nature of these interactions is important for a separate and distinct form of heating employs plasma formed by arc conditions of a high temperature, often more than 3000.degree. F., and at much reduced pressures or vacuum conditions. For instance, refer to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, Supplementary Volume, pages 599-608, Plasma Technology. In microwave technology, as applied in the subject invention, neither condition is present and therefore no plasmas are formed.
Microwaves lower the effective activation energy required for desirable chemical reactions since they can act locally on a microscopic scale by exciting electrons of a group of specific atoms in contrast to normal global heating which raises the bulk temperature. Further this microscopic interaction is favored by polar molecules whose electrons become easily locally excited leading to high chemical activity; however, nonpolar molecules adjacent to such polar molecules are also affected but at a reduced extent. An example is the heating of polar water molecules in a common household microwave oven where the container is of nonpolar material, that is, microwave-passing, and stays relatively cool.
In this sense microwaves are often referred to as a form of catalysis when applied to chemical reaction rates. For instance, refer to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, Volume 15, pages 494-517, Microwave Technology.